Copolymers of aromatic vinyl compounds and conjugated diolefins having substantial increase in aromatic vinyl compound differential content

ABSTRACT

There is disclosed copolymers of aromatic vinyl compounds, e.g., styrene, and conjugated diolefins, e.g., butadiene, possessing a differential content of the aromatic vinyl compound such that in at least one of the end portions of the copolymer the differential content shows a sharp and substantial increase in the direction of the outer extremity of the end portion. Preferred copolymers are styrene-butadiene copolymers having a vinyl content of at least 30%. In special embodiments, the copolymers have a styrene content changing in a portion of no more than 5% of the copolymer chain from a first value to a second value, the second value being at least 25 percentage points greater than the first value, and the portion is present within a 10% terminal portion of the copolymer. The copolymers are useful in the tread portions of tires as tires containing such copolymers in the tread composition have improved rolling resistance and/or grip on wet road surfaces.

This is a division of application Ser. No. 239,514 filed Mar. 2, 1981now abandoned.

This invention relates broadly to elastomeric copolymers of conjugateddienes and aromatic vinyl compounds, having novel structures and toprocesses of making them, to unvulcanised and vulcanised elastomericcompositions containing them and to their use in tires, in particular inthe tread portion of tires. The invention is particularly concerned withcopolymers derived from a styrene and a butadiene, for example styreneand 1,3-butadiene, and it is with reference to such polymers that thedescription hereinafter particularly relates.

The novel structure of the copolymers of this invention can be definedwith reference to the distribution of the styrene or other aromaticvinyl compound in the copolymer molecule. For example, if the proportionof styrene in successive incremental portions of the molecule(differential styrene content) is plotted against total monomerconversion (expressed as a percentage of the monomers reacting to formcopolymer) there is obtained a line with represents the respectiveamounts of styrene present along the length of the molecule. By way ofcomparison, a graph of this sort in respect of a true random copolymerhaving, for example, an average styrene content of 23% by weight is astraight horizontal line indicating that the styrene content ofsuccessive incremental portions of the molecule is substantially 23%throughout.

We have found, according to the present invention, that the propertiesof wet grip and/or rolling resistance of tires whose tread comprises astyrene-butadiene copolymer are significantly improved if the copolymeris one having in at least one of its end portions of the molecule asignificant styrene-rich component or zone such as is shown in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings designated as PRINT 1, PRINT 2, PRINT 3, PRINT 4, PRINT 6,PRINT 7, PRINT 8, PRINT 9, PRINT 10, PRINT 11/12, PRINT 13, PRINT14,14A,15 & 16, PRINT 16B, PRINT 16C, PRINT 16D, PRINT 17, PRINT 18,PRINT 19, PRINT 20, PRINT 21, PRINT 22, PRINT 23, PRINT 24, PRINT 26,PRINT 27, PRINT 28, PRINT 35/36, and PRINT 37 correspond to theExperiments correspondingly numbered in the specification. There are noprint numbers 5, 25, and 29-34. The drawings show the differentialstyrene content of the copolymers produced in accordance with thecorrespondingly numbered experiments.

FIG. 1 illustrates Print 1.

FIG. 2 illustrates Print 2.

FIG. 3 illustrates Print 3.

FIG. 4 illustrates Print 4.

FIG. 5 illustrates Print 6.

FIG. 6 illustrates Print 7.

FIG. 7 illustrates Print 8.

FIG. 8 illustrates Print 9.

FIG. 9 illustrates Print 10.

FIG. 10 illustrates Print 11/12.

FIG. 11 illustrates Print 13.

FIG. 12 illustrates Print 14, 14A, 15 and 16.

FIG. 13 illustrates Print 16B.

FIG. 14 illustrates Print 16C.

FIG. 15 illustrates Print 16D.

FIG. 16 illustrates Print 17.

FIG. 17 illustrates Print 18.

FIG. 18 illustrates Print 19.

FIG. 19 illustrates Print 20.

FIG. 20 illustrates Print 21.

FIG. 21 illustrates Print 22.

FIG. 22 illustrates Print 23.

FIG. 23 illustrates Print 24.

FIG. 24 illustrates Print 26.

FIG. 25 illustrates Print 27.

FIG. 26 illustrates Print 28.

FIG. 27 illustrates Print 35/36.

FIG. 28 illustrates Print 37.

The styrene-rich component can be considered to have two dimensions:length (that is, the proportion it constitutes of the total length orsize of the molecule) and height (that is, the maximum differentialstyrene content in said end portion). In general, it appears that themore important feature is the maximum differential styrene contentoccurring in the end portion and especially where this maximum isreached over a shorter rather than a longer length of the end portion.

Accordingly, a first aspect of the present invention provides anelastomeric copolymer of an aromatic vinyl compound and a conjugateddiene, suitable for use in the tread portion of a pneumatic tire, saidcopolymer having a vinyl content (as herein defined) of at least 30% byweight and having a differential content of the aromatic vinyl compoundsuch that in at least one of its end portions said differential contentshows a sharp and substantial increase in the direction of the outerextremity of said end portion.

In a second aspect the invention provides an elastomeric copolymersuitable for use in the tread portion of a pneumatic tire, whosepercentage differential styrene content changes in a portion of not morethan 5% of the copolymer chain (as determined by monomer conversion)from a first value to a second value, said second value being at least25 percentage points greater than said first value; and said portionlies within a terminal 10% portion of the copolymer chain (as determinedby monomer conversion).

Copolymers of particular interest are those in which the percentagedifferential styrene content changes in a zone of not more than 21/2% ofthe copolymer chain (as determined by monomer conversion) from a firstvalue to a second value, said second value being at least 14 percentagepoints greater than said first value; and said zone lies within aterminal 10% portion of the copolymer chain (as determined by monomerconversion). The 5% or 21/2% portion or zone referred to above can, forexample, be a terminal portion of the copolymer chain.

As can be seen from the accompanying drawings, the styrene tail in someof the copolymers of the invention is most pronounced in a very smallportion of the copolymer chain, for example 1/2%, 11/2% or 2% of thechain (as determined by total monomer conversion).

The term "vinyl content" as used herein refers to that portion by weightof the 1,3 butadiene or other diene component of the copolymer which haspolymerised at the 1,2 positions. Where the diene is 1,3 butadiene the1,2-polymerisation results in the formation of pendant vinyl groups;where the diene is other than 1,3 butadiene corresponding pendant groupsare produced by 1,2-polymerisation.

The vinyl aromatic compound is normally a styrene or other mono-vinylaromatic compound, for example: styrene, 1-vinyl naphthalene,3,5-diethylstyrene, 4-n-propylstyrene, 2,4,6-trimethyl styrene, 4-phenylstyrene, 4-p-tolystyrene 3,5-diphenylstyrene,3-ethyl-l-vinylnaphthalene, 8-phenyl-l-vinyl naphthalene.

Where, for example, branching or cross-linking is desired apolyfunctional vinyl compound can be used. For example, suitablepolyfunctional vinyl compounds are divinyl compounds, for instancedivinyl benzene.

The conjugated diene is one capable of polymerisation with styrene atthe 1,2 positions and such that, when polymerised with styrene or otherselected aromatic vinyl compound or compounds, it provides a polymerhaving desired elastomeric properties. The diene can, for example, be abutadiene or pentadiene, for example: 1,3-butadiene 2-methyl-l,3butadiene (isoprene), 1,3-pentadienes (piperylenes),2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-pentadiene,2,3-dimethyl-1,3-pentadiene, 2-phenyl butadiene.

The styrene-butadiene copolymers of the invention can, for example, beones whose average styrene content is at least 10% (for example 15, 20,25, 30 or 40%) by weight of the copolymer. However, the invention alsoincludes copolymers whose average styrene content is less than 10% byweight, for example copolymers whose portion or portions other than thestyrene-rich portion or portions has little or no styrene. Such acopolymer is that of Experiment 20 below. Mixtures of aromatic vinylcompounds and/or mixtures of dienes can be used.

The copolymers can have at one or more of their extremities a block ofpolystyrene or polybutadiene, particularly the latter. A polybutadieneblock will normally reduce rolling resistance at some expense to wetgrip, and a polystyrene block will, to a lesser extent, enhance wet gripat expense to rolling resistance. Therefore a block can be used toprovide (or at least to assist in providing) a desired combination ofwet grip and rolling resistance. We have found, however, that in generaland polystyrene block should not have a molecular weight as much as50,000 when the molecular weight of the copolymer (excluding the blockor blocks) is 300,000.

The preferred size of any polybutadiene block is in the range from20,000 to 50,000 or, expressed as a percentage of a 300,000 molecularweight copolymer, 7% to 17% by weight. If a polystyrene block is usedfor its contribution to wet grip the size of the block is preferably inthe range 20,000-40,000, or expressed as a percentage of a 300,000copolymer, 7% to 13%.

The copolymers of the invention can be obtained by a solutionpolymerisation process using a suitable initiator, for example, alithium-hydrocarbon compound. Suitable monolithium initiators (providinglinear polymers) are, for example: methyl lithium, ethyl lithium,n-propyl lithium, isopropyl-lithium, n-butyl lithium, sec-butyl lithium,tert. butyl lithium, n-amyl lithium, isoamyl lithium, n-hexyl lithium,n-octyl lithium, phenyl lithium.

If it is wished to produce a double or branched polymer a dilithium orother polylithium initiator can be used, for example: tetramethylenedilithium, pentamethylene dilithium, hexamethylene dilithium,phenylethylene dilithium, tetra phenylethylene dilithium. Polylithiuminitiators containing more than two lithium atoms per initiatormolecule, are for example, those derived from lithium divinylbenzene ofisoprene; such compounds are those commercially available under thetrade names DILI-3 and DILI-1 (Lithium Corporation of America).

Suitable solvents (which can be two- or other multi-component solvents)for the reaction are, for example, alkanes, alkenes, cycloalkanes andcycloalkenes, for instance: benzene, toluene, xylenes, ethylbenzene,iso-butane, n-pentane, iso-pentane, n-heptane, iso-octane, n-decane,cyclopentane, methyl cyclopentane, dimethylcyclopentane, cyclohexane,methyl cyclohexane, dimethylcyclohexane, 1-butene, 2-butene, 1-pentene,2-pentene, cyclopentene.

Where it is wished to provide enhanced wet grip for tires, there ispreferable used in the polymerisation reaction a substance (structuremodifier) whose use results in an increased amount of the butadienebeing polymerised at the 1,2-positions. Such polymerisation results inthe formation of vinyl groups (or corresponding groups where otherconjugated dienes are used) which enhance wet grip of tires whose treadcontains the polymer. Examples of suitable modifiers are as follows:

(1) Ethers, thioethers, cyclic ethers, tertiary amines; for instancediethylether, dimethylether, tetra hydrofuran, dioxane, orthodimethoxybenzene, Monoglyme, Diglyme, triethylamine;

(2) Hexamethylphosphortriamide;

(3) Difunctional Lewis bases, for instance, tetramethyl,ethylenediamine;

(4) Organic potassium or sodium compounds, for instance potassium-tert.butoxide.

The use of structure modifiers such as those referred to in thepreceding paragraph results in 1,2-polymerisation at the expense of 1,4or other alpha-omega polymerisation. Furthermore, the copolymerisationreaction rate of styrene with butadiene increases resulting in a morerandom copolymerisation. Thus, by suitable choice of reaction conditionsit is possible to produce a copolymer having a content of butadiene,albeit small, extending as far as that portion of the polymer producedat or towards full conversion. By use of a suitable modifier nopolystyrene blocks normally form at the end of the molecule unlessadditional styrene is added.

Various means can be adopted to provide an enhanced and/or an increasingproportion of styrene in that portion of the copolymer formed towardsthe beginning and/or end of the reaction. One such means is by use of astructure modifier as previously discussed. A second means is by theaddition of one or more further portions of styrene (usually withbutadiene) during the reaction, either in a continuous, semi-continuousor stepwise manner. A suitable semi-continuous process is one in which,during the reaction, the concentration of reactants changes and/or oneor more of the other reaction conditions changes at sequential timeintervals. It will be seen that some of the processes of the Examplesare semi-continuous in that they employ a mixture of styrene andbutadiene in the reactor before adding the modifier, and a further oneor more portions of styrene is added subsequently. Such subsequentaddition can be at the same time as introduction of the modifier (or afirst portion thereof) into the reaction zone and/or at a later time.

Where said second means is used the proportion of styrene which is addedin subsequent additions can usefully be greater than that in the reactorinitially and one or more of the subsequent additions can be of styrenewithout butadiene.

Suitable temperatures for the reaction are, for example, in the rangefrom 20° to 70° or 80° C. Care has to be taken in using the highertemperatures to speed up the conversion rate because higher temperaturesfavour 1,4-polymerisation and thereby can lead to a reduction in vinylcontent below 30%.

If desired, the copolymers of the invention can be in the form ofcomposite molecules obtained by coupling together two or more smallercopolymer molecules. Accordingly, in a third aspect the inventionprovides an elastomeric copolymer obtained by coupling together two ormore copolymers of the first or second aspects of the invention. Thecoupling can be effected conveniently by use of a coupling agent; adifunctional coupling agent, for example, dibromoethane, provides alinear coupled copolymer, and a tri- or other poly- functional couplingagent, for example the tetra-functional compounds silicon tetrachloride(SiCl₄), diethyladipate (DEAP), dimethyladipate or stannic chloride(SnCL₄) provides a non-linear or branched coupled copolymer.

It is to be noted that the styrene content depicted in all theaccompanying drawings is that of uncoupled copolymer.

The coupling is normally preferably performed so that the couplingcopolymer molecules couple with one another at the, or an, end of themolecule not having a tail. Coupling normally is effected at the end ofthe copolymerisation reaction and therefore when the copolymer is onehaving an end tail the coupling results in the tail ceasing to be "free"but becoming linked to another copolymer molecule. Where the coupling isonly partly complete, for example at 50% of the theoretical amount, theproduct of the coupling reaction consists partly of coupled copolymerand partly of uncoupled copolymer and the product of such coupling whenused in tire treads provides, at least to come extent, a usefulcombination of wet grip and rolling resistance values, though increasingdegrees of coupling at end tails is detrimental to said combination ofvalues.

On the other hand, coupling of copolymers having begin tails does notreduce the amount of free tails and in consequence there is no suchdetriment to said combination of values. Therefore in general it ispreferred, when a coupled product is required, to use a begin tailcopolymer as precursor.

Another way of providing branched polymers is to use an initiator havingthree or more active functions.

The polymerisation process can be terminated by use of an end-stoppingagent; this can be a proton-releasing compound, for instance water, analcohol or an amine.

This invention is illustrated by the following Examples (also referredto below as "Experiments" ). These Examples describe the production ofcopolymers of styrene and 1,3 butadiene, some of which were subsequentlycoupled to produce coupled copolymers. The formulation of the copolymersin elastomer compositions and the use of those compositions in (model)tires to demonstrate wet grip and rolling resistance properties of theelastomers is also described.

It will be seen that all the processes described are solutionpolymerisation processes using a lithium hydrocarbon compound and astructure modifier/randomiser. In some Examples the conditions are suchthat an increasing proportion of styrene is present in the reactor inthe later part of the reaction; and in some Examples there is an excessof styrene in the reactor in the earlier part of the reaction, whichfalls towards the end of the reaction.

It is believed that the high proportion of styrene (in admixture withbutadiene) at the early or late stage or stages of the reaction isrelated to the superior properties shown by the product copolymers. Itwill be seen that in the Examples a very full conversion was obtained,and this is believed to be of particular importance especially where ahigh proportion of styrene is present towards the end of the reaction.

Experiments 1 to 4 and 6 to 10 below describe the production of ninecopolymers of styrene and 1,3-butadiene, two of which were subsequentlycoupled to produce coupled copolymers.

EXPERIMENT NO. 1

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentaine. Hereafter were added 69 gramsof styrene and 231 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/1). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, a sec. butyl lithium solution (33.3 ml of a 100 mmol/1solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 700 grams of astyrene/butadiene blend (weight ratio styrene/butadiene 23/77 was pumpedinto the reactor at a rate of 12 grams/min.

The temperature of the reactor contents was kept at 60° C. by cooling.

After the addition of these monomer quantities the polymerisationreaction was allowed to proceed for a further 60 minutes, by which amonomer conversion of >99.9% was obtained. Hereafter 0.5 grams ofmethanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

EXPERIMENT NOS. 2 AND 3

Were carried out as experiment No. 1, except that in experiment No. 2the amount of orthodimethoxybenzene (ODMB) added was 1.13 grams andexperiment No. 3 the amount of ODMB was 0.50 grams.

EXPERIMENT NO. 4

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentane. Hereafter were added 15 gramsof styrene, 85 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/1). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interferring with the polymerisation reaction had beendeactivated, a sec. butyl-lithium solution (33.3 ml of a 100mmol/1solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 400 grams of astyrene/butadiene blend (weight ration styrene/butadiene 15/85) waspumped into the reactor at a rate of 20 grams/min.

The temperature of the reactor contents were kept at 60° C. by cooling.

Immediately after the addition of monomers had been completed anotherportion of 500 grams of styrene/butadiene blend (weight ratiostyrene/butadiene 31/69) was pumped into the polymer solution likewiseat a rate of 20 grams/min.

After the addition of these monomer quantities the polymerisationreaction was allowed to proceed for a further 90 minutes, by which amonomer conversion of >99.9% was obtained. Hereafter 0.5 grams ofmethanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

EXPERIMENT NO. 6

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentane. Hereafter were added 115 gramsof styrene, 38.5 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/1). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, a sec. butyl lithium solution (33.3 ml of a 100 mmol/1solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 654 grams of astyrene/butadiene blend (weight ration styrene/butadiene 17.5/82.5) waspumped into the reactor at a rate of 43.6 grams/min. The temperature ofthe reactor contents was kept at 60° C. by cooling.

Immediately after the addition of monomers had been completed 192.5grams of butadiene was pumped into the polymer solution at a rate of2.00 grams/min.

After the addition of the last monomer quantity the polymerisationreaction was allowed to proceed for a further 40 minutes, by which amonomer conversion of >99.9% was obtained. Hereafter 0.5 grams ofmethanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

EXPERIMENT NO. 7

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentane. Hereafter were added 30 gramsof styrene, 70 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/1). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, a sec. butyllithium solution (33.3 ml of a 100mmol/1solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 900 grams of astyrene/butadiene blend (weight ratio styrene/butadiene 22.2/77.8) werepumped into the reactor at a rate of 15 grams/min. The temperature ofthe reactor contents was kept at 60° C. by cooling.

The polymerisation reaction was allowed to proceed for a further 90minutes, by which a monomer conversion of >99.9% was obtained. Hereafter0.5 grams of methanol were added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

EXPERIMENT NO. 8

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentane. Hereafter were added 115 gramsof styrene, 38.5 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/1). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, a sec. butyllithium solution (66.6 ml of a 100mmol/1solution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 654 grams of astyrene/butadiene blend (weight ratio styrene/butadiene 17.5/82.5) waspumped into the reactor at a rate of 81.7 grams/min. The temperature ofthe reactor contents were kept at 60° C. by cooling.

Immediately afterthe addition of monomershad been completed 192.5 gramsof butadiene was pumped into the polymer solution at a rate of 1.9grams/min.

After the addition of these monomer quantities the polymerisationreaction was allowed to proceed for a further 40 minutes, by which amonomer conversion of >99.9% was obtained. Hereafter 0.33 grams ofdiethyladipate was added to couple the polymer chains to a polymer witha branched structure of a fourfold increased molecular weight (couplingefficiency ˜50%). Next 0.5 grams of methanol was added to kill thepolymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

EXPERIMENT NO. 9

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentane. Hereafter were added 115 gramsof styrene, 38.5 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/l). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, a sec. butyllithium solution 66.6 ml of a 100 mmol/lsolution in cyclohexane) was added to initiate the polymerisation.

Simultaneously with the addition of the initiator 654 grams of astyrene/butadiene blend (weight ratio styrene/butadiene 17.5/82.5) waspumped into the reactor at a rate of 81.7 grams/min. The temperature ofthe reactor contents were kept at 60° C. by cooling.

Immediately after the addition of monomers had been completed 192.5grams of butadiene was pumped into the polymer solution at a rate of 1.9grams/min.

After the addition of these monomer quantities the polymerisationreaction was allowed to proceed for a further 40 minutes, by which amonomer conversion of >99.9% was obtained. Hereafter 0.66 grams ofdibromoethan was added to couple the polymer chains to a linear polymerof a twofold increased molecular weight (coupling efficiency ˜50%). Next0.5 grams of methanol was added to kill the polymerisation reaction.

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

EXPERIMENT NO. 10

A 10 1. stainless steel reactor was charged with 2000 grams ofcyclohexane and 2000 grams of isopentane. Hereafter were added 55 gramsof styrene, 44.5 grams of butadiene and 2.25 grams oforthodimethoxybenzene. The contents were brought to a temperature of 60°C. Next the impurities in the solvent/monomer blend, capable of reactingwith sec. butyllithium were deactivated by titration with a sec.butyllithium solution in cyclohexane (conc. 100 mmol/l). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, a sec. butyllithium solution 33.3 ml of a 100 mmol/lsolution in (cyclohexane) was added to initiate the polymerisation.

After the addition of these monomer quantities the polymerisationreaction was allowed to proceed for a further 60 minutes, by which amonomer conversion of >99.9% was obtained. Hereafter 0.5 grams ofmethanol was added to kill the polymerisation

Finally 0.5 grams of 2.6 di-tert. butylparacresol were added as astabiliser. The polymer was recovered from the solution by coagulationwith steam.

The Examples are accompanied by graph prints showing a plot ofpercentage differential styrene in the copolymer part of the moleculeagainst total monomer conversion (which corresponds to percentagemolecular size of the copolymer molecules). The differential styrenecontents at various conversions were calculated from thecopolymerisation kinetics of styrene and butadiene using the followingequation (1) ##EQU1## in which S1=weight fraction of styrene incopolymer

B1=weight fraction of butadiene in copolymer

S=weight fraction of styrene in monomer blend

B=weight fraction of butadiene in monomer blend

R1=reactivity ratio of styrene

R2=reactivity ratio of butadiene.

The styrene and butadiene weight fractions in the copolymer in relationto their weight fractions in the monomer blend were determined bypolymerising styrene and butadiene feeds of varying compositions toconversions below 5% and measuring the styrene and butadiene content ofthe resulting copolymers by infra red analysis.

Knowing this relation, R1 and R2 were calculated according to the methoddescribed by Fineman and Ross in J. Polymer Science, 5, (1950) page 259.

For example, under the polymerisation conditions applied in the majorityof the Examples, viz. at 60° C. in cyclohexane as a solvent and using amodifier (ODMB) concentration of 450 PPM, we measured for R1 and R2values of 0.73 and 1.40 for styrene and butadiene respectively. Thus,starting with a monomer feed of known composition one can calculate thecopolymer composition at the onset of copolymerisation (approaching zero0/0 conversion) with the aid of equation (1). The composition of polymerfractions formed at further points in polymerisation time or conversionscan then be calculated, using the equation, from the monomer compositionremaining after a preceding polymer fraction or increment has beenformed.

The graph prints are included in the accompanying drawings and arereferred to as Prints 1 to 4 and 6 to 10, corresponding to Experiments 1to 4 and 6 to 10, respectively.

In the prints total monomer conversion is referred to, for the sake ofconvenience, as "monomer conversion" the horizontal lines represent theaverage styrene content (%) of the polymer.

Referring to print 1 of the accompanying drawings, it will be seen thatthe molecule of the polymer of Example 1 can be regarded as beingcomposed of three portions: a first end portion comprising about 40% ofthe molecule, in which the styrene content is not greater throughoutthan the average styrene content of the copolymer (23%), rising fromabout 17% to 23%; a second portion comprising about 49% of the moleculein which the styrene content is substantially the same throughout as theaverage styrene content; and a second end portion comprising about 10%of the molecule, in which the styrene content is greater throughout thanthe average styrene content of the polymer. This last-mentioned portionhas a styrene-rich component or "tail" in which the styrene contentrises at an increasing rate from about the 90% monomer conversion marktowards 100%, and the styrene content at the outer end of the endportion (or later end in terms of the production of the polymer) reachesa maximum of 57%.

The various values for the sizes of portions of the molecule and thedifferential styrene contents thereof can be obtained by reading off therespective print. For example, referring again to Print 1, it will beseen that the differential styrene content rises in the last 5% portionof monomer conversion (i.e. from 95% to 100% conversion) from a first(threshold) value (T) to 27% to a maximum value (M) of 57% at 100%monomer conversion, a rise (M-T) of 30 percentage points; and that inthe last 21/2% portion of total monomer conversion the differentialstyrene content changes from a threshold value (T') of 31% to themaximum value of 57%, a rise (M-T') of 26 percentage points. The valuesM, T, (M-T), T' and (M-T') are given in Table B below.

Prints 2 and 3 show that the polymers of Examples 2 and 3 have astructure broadly similar to that of Example 1, having a first endportion, a middle portion and a second end portion, the last-mentionedportion have a styrene-rich component in which the styrene content risessteeply from the average styrene content.

From Print 2 it can be seen that the threshold value (T) of differentialstyrene content at 95% monomer conversion is 32%, the threshold value(T') at 971/2% monomer conversion is 39 and the maximum styrene contentis 72%. Thus, the differential styrene content rises by 40 percentagepoints over the last 5% portion of monomer conversion and by 33percentage points over the last 21/2% of monomer conversion.

In print 6 it is the first end portion (referred to herein as a "begintail") of the molecule that has the styrene-rich component, and thesecond or later end portion has a styrene content less that the averagestyrene content of the polymer; in between the styrene content issubstantially the same as the average styrene content of the polymer.

It will be seen that the differential styrene content of the copolymerof Experiment 6 changes over that portion of monomer conversion between0% and 5% conversion from a first (threshold) value (T) of 46 to asecond (maximum) value (M) of 71% at 0% monomer conversion, that is, achange (M-T) of 25 percentage points; and that over the terminal 21/2%zone of said 5% portion the differential styrene content changes from afirst (threshold) value (T') of 56% to the second (maximum) value (M) of71% at 0%, that is, a change (M-T') of 15 percentage points.

The respective values of M, T, T', (M-T) and (M-T') for all of thecopolymers of Experiments 1-10 are given in Table B from which it willbe seen that all the values for (M-T) are 25 percentage points or moreand that all the values for (M-T') are 14 percentage points or more. Inthat Table under the heading "Tail" it is stated whether the copolymertail occurs at or towards the beginning of the monomer conversion (a"begin tail") or at or towards the end of the monomer conversion (an"end tail").

Prints 8 and 9 illustrate the polymers of Experiments 8 and 9 which areformed as the result of coupling a polymer obtained substantially as inExperiment 6, and it will be seen that those two prints are similar tothat of print 6. The polymer of print 8 is believed to be branched andcomprises four polymer linear chains coupled together whereas that ofprint 9 is believed to be linear and to comprise two polymer linearchains coupled together.

As is made clear from the prints, reference above to certain percentageproportions of the "length of the molecule" are derived from thecorresponding figures for monomer conversion, and it is not to beinferred that the molecules of any given polymer are all of the samelength.

It will be seen from the prints that in the various end portions thedifferential styrene content rises rapidly towards a high valuecorresponding to approximately 100% (or 0%) of the length of the polymermolecule as measured in terms of monomer conversion; and in some of thepolymers the increase is particularly great over the last few (forexample, one, two or three) percent of the polymer length.

It will be seen from Table A that the vinyl content (that is the amountof the butadiene component of the polymer that has polymerised at the1,2-positions) is in each case 30% or more and that most of the valueslie in the range 40 to 50%, especially 45 to 50%. The styrene content ofeach polymer is in the range 20 to 30%.

                  TABLE A                                                         ______________________________________                                                Styrene Content %                                                                           Vinyl                                                   Experiment                                                                            (Average)     Content % Chain Structure                               ______________________________________                                        1       23            48        L                                             2       23            42        L                                             3       23            30        L                                             4       23            49        L                                             6       23            47        L                                             7       23            47        L                                             8       23            47        B                                                                             (DEAP-coupled)                                9       23            47        L                                                                             (DBE-coupled)                                 10      23            47        L                                             ______________________________________                                         Notes:                                                                        DEAP = diethyl adipate                                                        DBE = dibromoethane                                                           L = linear                                                                    B = branched                                                             

                  TABLE B                                                         ______________________________________                                        Experi-  Threshold    Maxi-        Thres-                                     ment     (T)          mum          hold                                       No.   Tail*  at 5%    at 95%                                                                              (M)   M-T  (T')  M-T'                             ______________________________________                                        1     E      --       27    57    30   31    26                               2     E      --       32    72    40   39    33                               3     E      --       36    72    36   46    26                               4     E      --       37    97    60   41    56                               6     B      46       --    71    25   56    15                               7     E      --       27    56    29   31    25                               8     B      46       --    71    25   57    14                               9     B      46       --    71    25   57    14                               10    E      --       43    79    36   48    31                               11,12 E      --       37    95    58   42    53                               13    E      --       38    95    57   42    53                               14,14A                                                                              E      --       37    95    58   43    52                               15,16                                                                         16B   B      31       --    82    51   38    44                               16C   B      44+      --    94    50   71    23                               16D   B      29       --    83    54   35    48                               17    E      --       34    97    63   38    59                               18    E      --       39    97    58   34    63                               19    E      --       24    97    73   27    70                               20    E      --       13    94    81   19    75                               21    E      --       37    95    58   42    53                               22    E      --       41    97    56   46    51                               23    E      --       47    97    50   51    46                               24    E      --       51    98    47   56    42                               26    B      19++     --    92    73   27    65                               27    B      21       --    48    27   21    27                               28    B      21+++    --    47    26   21    26                               35/36 B      30       --    83    53   38    45                               37    B      37       --    83    46   50    33                               ______________________________________                                         +based on 5% band at 3-8% conversion                                          ++based on 5% band at 21/2-71/2% conversion                                   +++based on 5% band at 1/2-51/2% conversion                              

The compositions of Examples 1 to 10 have been tested to assess theirwet grip and rolling resistance properties on a road surface. Each ofthose compositions was used as the tread compound of model tires of size2.25-8 (dimensions in inches). These model tires were subjected to twotests as follows.

Grip on a wet Delugrip road surface (Delugrip is a Registered TradeMark) was measured using the variable speed internal drum machine(VSIDM) described in a paper by G. Lees and A. R. Williams in Journal ofthe Institute of the Rubber Industry, Vol. 8, No. 3, June 1974.Measurements of the wet grip were made for locked wheel slidingfriction. Rolling resistance was measured using the rotary power lossmachine described in Transactions of the Institution of Rubber Industry34, No. 5, October 1958, applying Equation 3.1 given below.

The results obtained are shown in Table C below. Equation 3.1, asapplied to SP4 steel radial tires of Dunlop Limited, referred to in thetable is as follows: ##EQU2## where E" and E"/(E*)² have the followingmeanings: E"=loss modulus expressed in MPa

E*=complex modulus expressed in MPa

The expression E"/(E*)² is expressed in GN/m².

                  TABLE C                                                         ______________________________________                                                     Wet Grip  Rolling Resistance                                                  Sliding   calculated according                                   Example No.  Friction  to Equation 3.1                                        ______________________________________                                        Comparisons:                                                                  S-1502       100       100                                                    S-1712       116       101                                                    OEP/LTP      112       115                                                    1.           108       93                                                     2.           105       91                                                     3.           104       90                                                     4.           116       93                                                     6.           105       94                                                     7.           109       92                                                     8.           110       99                                                     9.           108       95                                                     10.          108       95                                                     ______________________________________                                    

It will be seen from Table C that the tires tested have a goodcombination of wet grip and rolling resistance. It is believed that theend portion of the polymer molecule having the styrene-rich componentmakes a major contribution to the good wet grip and that the remainderof the molecule having a substantial vinyl content and free from astyrene-rich component makes a major contribution to the good rollingresistance.

The production of further copolymers of the invention, their formulationin elastomeric compositions and use of those compositions in the treadportion of tires is described below.

Abbreviations used have the following meanings:

S=styrene

B=butadiene

S/B=mixture of styrene and butadiene

SBR=styrene-butadiene copolymer

ODMB=ortho-dimethoxybenzene

DEAP=diethyl adipate

DVB=divinyl benzene

S-BuLi=secondary butyl lithium

Diglyme=diethylene glycol dimethylether

PS=polystyrene

MW=molecular weight

B=branched molecule

L=linear molecule

min=minutes

BC=before coupling.

Unless otherwise stated the ratios and percentages referred to in theExperiments are by weight.

Experiments 11, 12 and 13 relate to copolymers having small polystyreneblocks at one or two ends of the molecule.

Experiments 14 to 15C relate to copolymers having a branched structure.

Experiments 17 to 24 relate to copolymers having different proportionsof bound styrene in the main chain of the molecule.

Experiments 26, 27 and 28 relate to copolymers subsequently converted tobranched form by coupling using DEAP.

There now follows descriptions of the production of the copolymers.

EXPERIMENT 11

A styrene-butadiene copolymer was produced using the ingredients andconditions referred to in Table 1A below. The procedure used was asfollows.

A 10 liters stainless steel reactor was charged with 4000 grams ofcyclohexane. Thereafter were added a first (1) batchwise charge ofmonomer and the ODMB, and the contents were brought to a temperature of60° C. Next, the impurities in the solvent/monomer blend, capable ofreacting with sec. butyllithium were deactivated by titration with asec. butyllithium solution in cyclohexane (conc. 100 mmol/l). After atemperature rise of 0.5° C. was observed, indicating that all impuritiescapable of interfering with the polymerisation reaction had beendeactivated, the sec. butyllithium solution (33.3 ml of a 100 mmol/lsolution in cyclohexane) was added to initiate the polymerisation.

After 10 minutes the second (2) batchwise charge of monomers was added,immediately followed by the first continuous addition of monomer over aperiod of 25 minutes. The temperature of the reactor contents was keptat 60° C. by cooling.

Immediately after the first continuous addition of monomers had beencompleted the second continuous addition of monomers was pumped into thereactor during 25 minutes. Thereafter, the polymerisation reaction wasallowed to proceed for a further 60 minutes by which time a monomerconversion of more than 99.9% had been obtained. Then 0.5 gram ofmethanol was added to kill the polymerisation reaction. 0.5 gram of 2,6di-tert. butylparacresol was added as a stabiliser. The polymer wasrecovered from the solution by coagulation with steam.

EXPERIMENT 12

The procedure of Experiment 11 was followed except that there was onlyone batchwise charge of monomers, which immediately after initiation ofthe polymerisation was followed by three successive, continuousadditions of monomer(s) over a period of 55 min.

EXPERIMENT 13

The procedure of Experiment 11 was followed, except that there was athird continuous addition, immediately after the second continuousaddition had been completed.

EXPERIMENTS 14, 14A, 15, 16, 16B, 16C AND 16D

Seven branched styrene-butadiene copolymers were produced using theingredients and conditions referred to in Table IIA. The procedure usedwas that described in Experiment 12 except that there were only twocontinuous additions and, in Experiments 14, 14A, 15, 16B, 16C and 16D,there was added to the reactor after 90 or 120 minutes a coupling agentas indicated in Table IIA. Use of the coupling agent (0.08 mmol) DEAP orSnCL₄) resulted in the copolymer chains being partly (about 50%) coupledto give a polymer with a branched structure having a fourfold molecularweight.

In experiment 16 coupling or branching was obtained by use of DVB in athird addition stage.

After the coupling methanol and 2.6 di-tert. butylparacresol were addedas described in Experiment 11.

EXPERIMENTS 17 TO 24

Further styrene-butadiene copolymers were produced using the ingredientsand conditions referred to in Tables IIIA and IVA. The procedure wasthat described in Experiment 11 except that there was only one batchwisemonomer charge and only one continuous monomer charge.

EXPERIMENTS 26, 27 AND 28

Three further styrene-butadiene copolymers were produced using theingredients and conditions referred to in Table VA. The procedures usedwere as follows:

EXPERIMENT 26

Begin tail: Amount: 5% of total polymer: styrene/butadiene ratio 90/10

Main chain: Amount: 95% of total polymer: styrene/butadiene ratio19.5/80.5

A stainless steel reactor of 10 liters capacity was charged with 4000grams of cyclohexane, 45 grams of styrene, 5 grams of butadiene and 2.25grams of ODMB. The contents were brought to a temperature of 60° C. byexternal heating of the reactor. Next the polymerisation of the monomerswas initiated by addition of 66.6 ml of a solution of S-BuLi incyclohexane of a concentration of 100 mmol/liter. After a polymerisationtime of 30 minutes during which virtually all of the monomers wereconverted into a styrene-butadiene copolymer, a blend of 185 grams ofstyrene and 765 grams of butadiene was added in 20 minutes. During thisaddition the temperature of the reactor contents was kept at 60° C.After the monomers had been added the polymerisation reaction wasprolonged for another 60 minutes at 60° C. after which 0.30 grams ofDEAP were added in order to couple the living linear polymer chains intoa polymer of radial structure. Next 0.5 grams of 2.6 di-tertiarybutylparacresol were added as a stabiliser, after which the copolymerwas recovered from the solution by steam coagulation and drying.Characteristics of the polymer are shown in Table VB.

EXPERIMENT 28

Begin tail: Amount: 2% of total polymer; styrene/butadiene ratio 40/60

Main chain: Amount: 98% of total polymer; styrene/butadiene ratio22.7/77.3

The reactor of Experiment 26 was charged with 4000 grams of cyclohexane,8 grams of styrene, 12 grams of butadiene and 2.25 grams of ODMB. Themonomers were polymerized at 60° C. by addition of 66.6 ml of a S.BuLisolution in cyclohexane of 100 mmol/liter. After 30 minutes a blend of222 grams of styrene and 758 grams of butadiene were fed into thereaction zone in 20 minutes. Thereafter the same procedure was followedas in Experiment 26. Characteristics of the polymer are likewise shownin Table VB.

EXPERIMENT 27

Begin tail: Amount: 5% of total polymer; styrene/butadiene ratio 40/60

Main chain: Amount: 95% of total polymer; styrene/butadiene ratio 40/60and ratio 22.1/77.9.

The reactor of Experiment 26 was charged with 4000 grams of cyclohexane,20 grams of styrene, 30 grams of butadiene and 2.25 grams of ODMB.Polymerisation was initiated by addition of 66.6 ml of a S-BuLi solutionin cyclohexane of 100 mmol/liter. After 30 minutes a blend of 210 gramsof styrene and 740 grams of butadiene were fed into the reaction zone in20 minutes. Thereafter the same procedure was followed as in Experiment26. Characteristics of the polymer are shown in Table VB.

                                      TABLE 1A                                    __________________________________________________________________________              Batchwise Charged                                                                     Modifier                                                                      ppm on     Continuously Charged                                                                           Polymer-                        Exp.                                                                             Sample         total BuLi 1st   2nd   3rd  isation                         No.                                                                              No.    Monomer intake                                                                              m · mol                                                                   Addition                                                                            Addition                                                                            Addition                                                                           conditions                      __________________________________________________________________________    11 WB 223/224                                                                           (1)16.7 g S                                                                           450 ODMB                                                                            3.33 435 g S/B                                                                           500 g S/B                                                                           --   120 min.                                  after 10 min.      ratio 11.7/                                                                         ratio 31/69                                                                              60° C.                             (2) 48.3 g S/B     88.3 in 25                                                                          in 25 min.                                           ratio 11.7/88.3    min.                                             12 WB 225/226                                                                           48.3 g S/B                                                                            450 ODMB                                                                            3.33 435 g S/B                                                                           500 g S/B                                                                           16.79 S                                                                            120 min.                                  ratio 11.7/88.3    ratio 11.7/                                                                         ratio 31/69                                                                         in   60° C.                                                88.3 in 25                                                                          in 25 min.                                                                          5 min.                                                            min.                                             13 WB 231/232                                                                           (1) 16.7 g S                                                                          450 ODMB                                                                            3.33 420 g S/B                                                                           500 g S/B                                                                           16.79 S                                                                            120 min.                                  after 10 min.      ratio 8.4/                                                                          ratio 31/                                                                           in   60° C.                             (2) 46.6 g S/B     91.6 in                                                                             69 in 25                                                                            5 min.                                         ratio 8.4/91.6     25 min.                                                                             min.                                       __________________________________________________________________________     Polymerisation Data                                                           Solvent: Cyclohexane, 4000 g                                                  Amount of Polymer: 1000 g                                                     Final Solids Content: 20% w                                              

                                      TABLE 11A                                   __________________________________________________________________________    Batchwise Charged                                                                      modifier                                                                      ppm on    Continuously Charged                                                                             Polymer-                                                                              Coupling                        Exp.     total BuLi                                                                              1st    2nd   3rd   isation Agent                           No.                                                                              Monomer                                                                             intake                                                                              mmol                                                                              Addition                                                                             Addition                                                                            Addition                                                                            conditions                                                                            mmol                            __________________________________________________________________________    14 150 g S/B                                                                           450 ODMB                                                                            3.33                                                                              350 g S/B                                                                            500 g S/B    90 min. 60° C.                                                                DEAP                               ratio 15/       ratio  ratio 31/69                                            85              15/85 in                                                                             in 25 min.                                                             25 min.                                                    14A                                                                              150 g S/B                                                                           150   3.5 350 g S/B                                                                            500 g S/B    90 min. 50° C.                                                                DEAP                               ratio Diglyme   ratio 15/85                                                                          ratio 31/69                                            15/85           in 25 min.                                                                           in 25 min.                                          15 150 g S/B                                                                           450 ODMB                                                                            5.5 350 g S/B                                                                            500 g S/B    90 min. 60° C.                                                                SnCl.sub.4                         ratio           ratio 15/85                                                                          ratio 31/69                                            15/85           in 25 min.                                                                           in 25 min.                                          16 150 g S/B                                                                           450 ODMB                                                                            3.33                                                                              350 g S/B                                                                            500 g S/B                                                                           3.33 mmol                                                                           120 min. 60° C.                                                                --                                 ratio 15/85     ratio 15/85                                                                          ratio 31/69                                                                         DVB                                                              in 25 min.                                                                           in 25 min.                                          16B                                                                              23 g S                                                                              450 ODMB                                                                            8.0 900 g S/B                                                                            77 g B      120 min. 60° C.                                                                DEAP                                               ratio 23/77                                                                          in 20 min.                                                             in 80 min.                                                 16C                                                                              94.3 g S                                                                            150   6.7 After 5 min.                                                                         70 g B      100 min. 50° C.                                                                DEAP                               5.0 g B                                                                             Diglyme   830 g S/B                                                                            in 35 min.                                                             ratio 16.3/                                                                   83.7 in 40 min.                                            16D                                                                              23.0 g S                                                                            450 ODMB                                                                            6.7 After 1 min.                                                                         73.2 g B    120 min. 60° C.                                                                DEAP                               g B 3.8         900 g S/B                                                                            in 30 min.                                                             ratio 23/77 in                                             __________________________________________________________________________     POLYMERISATION DATA                                                           Solvent: cyclohexane, 4000 g                                                  Final Solids content: 20% w                                                   Amount of polymer: 1000 g                                                

                                      TABLE 111A                                  __________________________________________________________________________             Batchwise Charged                                                                   modifier                                                                      ppm on     Continuously Charged                                                                          Polymer-                            Exp.                                                                             Sample      total BuLi 1st   2nd  3rd  isation                             No.                                                                              No.   Monomer                                                                             intake                                                                              m · mol                                                                   Addition                                                                            Addition                                                                           Addition                                                                           conditions                          __________________________________________________________________________    17 D250/251                                                                            900 grams                                                                           450 ODMB                                                                            3.33 After 15                                                                            --   --   120 min.                                     S/B ratio        min 100         60° C.                                22.1/77.9        grams S/B                                                                     ratio 31/69                                                                   in 15 min.                                          18 D252/253                                                                            900 grams                                                                           450 ODMB                                                                            3.33 after 15                                                                            --   --   120 min.                                     S/B grams        min 100         60° C.                                16.6/83.4        grams S/B                                                                     ratio 31/69                                                                   in 15 min.                                          19 D255/256                                                                            900 grams                                                                           450 ODMB                                                                            3.33 after 15                                                                            --   --   120 min.                                     S/B ratio        min 100         60° C.                                6.6/93.4         grams S/B                                                                     ratio 31/69                                                                   in 15 min.                                          20 D257/259                                                                            900 grams                                                                           450 ODMB                                                                            3.33 after 15                                                                            --   --   120 min.                                     butadiene        min 100         60° C.                                                 grams S/B                                                                     ratio 31/69                                                                   in 15 min.                                          __________________________________________________________________________     POLYMERISATION DATA                                                           Solvent: Cyclohexane, 4000 g.                                                 Amount of Polymer: 1000 g                                                     Final Solids content: 20% w.                                             

                                      TABLE 1VA                                   __________________________________________________________________________             Batchwise Charge                                                                    Modifier ppm                                                                              Continuously Charged                                                                          Polymer-                           Exp.                                                                             Sample      on total                                                                             BuLi 1st     2nd     isation                            No.                                                                              No.   Monomer                                                                             intake m · mol                                                                   Addition                                                                              Addition                                                                              conditions                         __________________________________________________________________________    21 WB236/237                                                                           100 grams                                                                           450    3.33 After 1 min.                                                                          After 21 min.                                                                         120 min.                                    S/B ratio         400 grams S/B                                                                         500 grams S/B                                                                         60° C.                               15/85             ratio 15/85                                                                           ratio 31/69                                                           in 20 min.                                                                            in 25 min.                                 22 WB242/243                                                                           100 grams                                                                           450    3.33 After 1 min.                                                                          After 21 min.                                                                         120 min.                                    S/B ratio         400 grams S/B                                                                         500 grams                                                                             60° C.                               10/90             ratio 10/90                                                                           S/B ratio                                                             in 20 min.                                                                            36/64 in                                                                      25 min.                                    23 WB244/245                                                                           100 grams                                                                           450    3.33 After 1 min.                                                                          After 21 min.                                                                         120 min.                                    S/B ratio         400 grams S/B                                                                         500 grams S/B                                                                         60° C.                               5/95              ratio 5/95 in                                                                         ratio 41/59                                                           20 min. in 25 min.                                 24 WB246/247                                                                           100 grams                                                                           450    3.33 After 1 min.                                                                          After 21 min.                                                                         120 min.                                    S/B ratio         400 grams                                                                             500 grams                                                                             60° C.                               0/100             S/B ratio                                                                             S/B ratio                                                             0/100 in                                                                              46/54 in                                                              20 min. 25 min.                                    __________________________________________________________________________     POLYMERISATION DATA                                                           Solvent: cyclohexane 4000 g                                                   Final Solids Content: 20% w                                                   Amount of Polymer: 1000 g                                                

                                      TABLE VA                                    __________________________________________________________________________            Batchwise Charged                                                                                  2nd step                                                 1st step                   Continuously                                                  ODMB      ODMB  charged in the                                                (modifier)                                                                              (modifier)                                                                          second step                                                                            Polymerisation conditions                 Monomer    ppm   sec.                                                                              ppm on     Buta-                                                                             time min.                                                                            temp. °C.           Exp. No.                                                                              Styrene                                                                            Butadiene                                                                           on total                                                                            BuLi                                                                              total Styrene                                                                            diene                                                                             1st                                                                              2nd 1st                                                                              2nd                     & Sample                                                                              grams                                                                              grams intake                                                                              mmol                                                                              intake                                                                              grams                                                                              grams                                                                             st.                                                                              st. st.                                                                              st.                     __________________________________________________________________________    26(D292/293)                                                                          49.5 5.5   450   7.33                                                                              450   204  842 30 60  50 50                      27(PS241)                                                                             8.8  13.2  450   7.33                                                                              450   244  847 30 60  50 50                      28(PS242)                                                                             22   33    450   7.33                                                                              450   231  814 30 60  50 50                      __________________________________________________________________________     POLYMERISATION DATA                                                           Solvent 1st step: Cyclohexane 2.340 kg                                        Solvent 2nd step: Cyclohexane 2.340 kg                                        Final Solids Content: 19% w                                                   Amount of Polymer: 1100 grams                                            

Salient data concerning the structure of the copolymers of theExperiments, is set out in Tables 1B, 11B, 111B, IVB and VB shown below.

In the tables:

the Mooney values are ML 1+4 100° C.;

the IR analysis values are in weight %

Q=Mw/Mn.

                                      TABLE 1B                                    __________________________________________________________________________                GPC Analysis IR Analysis                                          Exp.                                                                             Sample   app Mw × 10.sup.3                                                                    Butadiene part                                       No.                                                                              No.   Peak                                                                             Mw  Mn  --Mw/--Mn                                                                          vinyl                                                                            CIS                                                                              trans                                                                            Styrene                                                                            Mooney                                 __________________________________________________________________________    11 WB223/224                                                                           450                                                                              453 252 1.80 46 19 35 24.6 106                                    12 WB225/226                                                                           390                                                                              377 220 1.72 46 19 35 23.9 90                                     13 WB231/232                                                                           400                                                                              383 248 1.55 49 15 36 24.3 98                                     __________________________________________________________________________

                                      TABLE 11B                                   __________________________________________________________________________                  Chain   GPC analysis                                                                             Coup-                                                                             IR analysis on                           Exp.                                                                             Sample                                                                              Structure                                                                          Struct-                                                                            Peak                                                                             App Mw × 10.sup.3                                                               --Mw/                                                                            ling                                                                              butadiene part                                                                           Sty-                          No.                                                                              No.   Modifier                                                                           ure  BC Mw  Mn  --Mn                                                                             %   Vinyl                                                                             Cis                                                                              Trans                                                                             rene                                                                             Mooney                     __________________________________________________________________________    14 GM38/40                                                                             ODMB B/DEAP                                                                             510                                                                              695 379 1.83                                                                             19  46  19 35  23.7                                                                             108                           /41                                                                        14A                                                                              GM43/46                                                                             Diglyme                                                                            B/DEAP                                                                             450                                                                              736 210 3.5                                                                              30  70  10 20  22.8                                                                             114                           /47                                                                        15 PS210/211                                                                           ODMB B/SnCl4                                                                            325                                                                              612 256 2.38                                                                             35  50  17 33  24.1                                                                              94                        16 PS208/209                                                                           ODMB B/DVB                                                                              460                                                                              558 252 2.21                                                                             10  50  17 33  24.8                                                                             119                                      copol                                                           16B                                                                              WRC5801                                                                             ODMB B/DEAP                                                                             190                                                                              441 147 3.01                                                                             56  47  18 35  24.9                                                                              56                        16C                                                                              PS/214/                                                                             Diglyme                                                                            B/DEAP                                                                             320                                                                              626 139 4.5                                                                              42  66  11 23  22.9                                                                              78                           216                                                                        16D                                                                              WRC5802                                                                             ODMB B/DEAP                                                                             183                                                                              432 278 1.55                                                                             63  47  17 36  24.8                                                                              47                        __________________________________________________________________________

                                      TABLE 111B                                  __________________________________________________________________________             GPC Analysis    IR Analysis                                          Exp.     App Mw × 10.sup.3                                                                       Butadiene part                                       No.                                                                              Sample No.                                                                          Peak                                                                              --Mw                                                                              --Mn                                                                             --Mw/--Mn                                                                          Vinyl                                                                             CIS                                                                              Trans                                                                             Styrene                                                                            Mooney                               __________________________________________________________________________    17 D250/251                                                                            410 403 258                                                                              1.56 48  17 35  24.7  98                                  18 D252/253                                                                            470 522 314                                                                              1.66 48  17 35  20.1 100                                  19 D255/256                                                                            460 458 313                                                                              1.47 49  17 34  10.2 102                                  20 D257/259                                                                            490 503 308                                                                              1.63 51  17 32   3.4 110                                  __________________________________________________________________________

                                      TABLE 1VB                                   __________________________________________________________________________             GPC Analysis    IR Analysis                                          Exp.                                                                             Sample                                                                              APP --Mw × 10.sup.3                                                                     Butadiene part                                       No.                                                                              No.   Peak                                                                              --Mw                                                                              --Mn                                                                             --Mw/--Mn                                                                          Vinyl                                                                             CIS                                                                              Trans                                                                             Styrene                                                                            Mooney                               __________________________________________________________________________    21 WB236-237                                                                           500 503.3                                                                             323.2                                                                            1.56 48  17 35  25.4 107                                  22 WB242/243                                                                           440 433.3                                                                             282.7                                                                            1.53 47  17 36  26.1 114                                  23 WB244-245                                                                           470 473.9                                                                             285.0                                                                            1.66 48  17 35  24.7 110                                  24 WB246-247                                                                           500 498.5                                                                             318.0                                                                            1.57 47  19 34  24.6 104                                  __________________________________________________________________________

                                      TABLE V B                                   __________________________________________________________________________                             Main Chain Overall                                          Initial part of polymer ("Begin tail")                                                          Amount on                                                                           Styrene/                                                                           Styrene                                   Exp. No.                                                                             Amount on         total butadiene                                                                          content %                                 (& Sample                                                                            total polymer                                                                         Styrene/but-                                                                            polymer                                                                             ratio                                                                              (IR                                       No.)   %       adiene ratio W/W                                                                        %     W/W  Analysis)                                 __________________________________________________________________________    26(D292/293)                                                                         5       90/10     95    19.5/80.5                                                                          23.5                                      27(PS241)                                                                            5       40/60     95    22.1/77.9                                                                          23.2                                      28(PS242)                                                                            2       40/60     98    22.7/77.3                                                                          23.1                                      __________________________________________________________________________    CHARACTERISTICS OF SBR POLYMERS                                                      Structure of butadiene                                                                   GPC Analysis                                                Exp. No.                                                                             portion of copolymer                                                                     App. peak                                                   (& Sample     Trans                                                                             MW × 10.sup.3                                                                              Mooney                                   No.)   1,2                                                                              cis 1,4                                                                           1,4 BC  CE  --M.sub.W                                                                        --M.sub.N                                                                        --MW/--Mn                                                                          Viscosity                                __________________________________________________________________________    26(D292/293)                                                                         50.9                                                                             13.6                                                                              35.5                                                                              230 76  535                                                                              271                                                                              1.97 74                                       27(PS241)                                                                            50.6                                                                             15.7                                                                              33.7                                                                              219 66  524                                                                              262                                                                              2.00 66                                       28(PS242)                                                                            52.5                                                                             13.6                                                                              33.9                                                                              211 70  555                                                                              262                                                                              2.12 65                                       __________________________________________________________________________

By use of the reaction of the various processes of the Experiments 11 etseq there have been produced graphs showing a plot of % differentialstyrene in the copolymer part of the molecule against % monomerconversion (which corresponds to % molecular size of the polymermolecules). Thus, for example, Prints 11-13 do not show the polystyreneblock present at an end of the molecule. These graphs are included inthe accompanying drawings and bear numbers corresponding to the numbersof the Experiments; for example "Exp.11/12"refers to the structure ofthe products of Experiments 11 and 12 and "Exp. 13" refers to theproduct of Experiment 13.

Referring to print "Exp. 17" of the accompanying drawings by way ofexample, it will be seen that the molecule of the polymer of Experiment17 can be regarded as being composed of two portions: a first or mainchain portion comprising about 90% of the molecule, over which thestyrene content rises from 17% at one end (the "begin" end) of themolecule to 30% at the end of said first portion, and a second or endportion comprising the remaining 10% or thereabouts of the molecule.This last-mentioned portion has a styrene-rich component or portion inwhich the styrene content rises rapidly from about 30% at the 90%monomer conversion mark towards 100%, and the styrene content at theouter end of the end portion (or later end in terms of the production ofthe polymer) is 97%.

The various values for the sizes of portions of the molecule and thedifferential styrene contents thereof can be obtained by reading off therespective print.

On each of the prints there has been marked a first (threshold) value(T) at one end of a portion of the graph corresponding to 5% of themonomer conversion, and a second (maximum) value (M) within said 5%portion. In all the prints except those numbered 16C, 26, 27 and 28 said5% portion extends from 0% to 5% or from 95% to 100% monomer conversion;in Print 16C said 5% portion extends from 3 to 8% monomer conversion; inPrint 26 it extends from 21/2% to 71/2%; and in Print 28 it extends from1/2 to 51/2%. In Print 27 only a 21/2% portion of the monomer conversionis shown. In all prints except Print 27 the respective terminal 5%portion of monomer conversion has been bisected to divide said portioninto two 21/2% zones of monomer conversion, which illustrates the changein differential styrene content over those two zones. In all instances(including Print 27) the styrene content rises by more than 14percentage points.

It is to be noted that the prints show differential styrene content ofcopolymer only; where there is a polystyrene component it is not shownon the print. In Print 16C the horizontal line at 94% styrene contentrefers to copolymer.

As is made clear from the prints, reference above to certain percentageproportions of the "length of the molecule" are derived from thecorresponding figures for monomer conversion, and it is not to beinferred that the molecules of any given polymer are all of the samelength.

It will be seen from the tables that the vinyl content (that is theamount of the butadiene component of the polymer that has polymerised atthe 1,2-positions) is in each case 30% or more and that most of thevalues lie in the range 40 to 50%, especially 45 to 50%. The styrenecontent of most of the polymers is in the range 20 to 30%, especially 25to 30%, though some values are about 10% or less and some values above50%. In general, most values are at least 30%.

Each of the copolymers has been formulated in an elastomer compositionhaving in each case the following constitution.

    ______________________________________                                                            Parts by weight                                           ______________________________________                                        Copolymer             100                                                     Sulphur               1.75                                                    Accelerator - CBS     1                                                       (cyclohexylbenzthiazyl sulphenamide)                                          Carbon black N 375    50                                                      Antioxidant BLE 75    2                                                       Zinc oxide            3                                                       Stearic acid          1                                                       ______________________________________                                    

The elastomeric compositions were vulcanised at 140° C. for 60 minutesin a steam autoclave, using a 15 minute rise to temperature.

The elastomeric compositions have been tested to assess their wet gripproperties on a road surface. Each of those compositions was used as thetread compound of model tires of size 2.25-8 (dimensions in inches).These model tires were subjected to the following tests to determine wetgrip. Grip on a Delugrip road surface (Delugrip is a Registered TradeMark) was measured using the variable speed internal drum machine(VSIDM) described in a paper by G. Lees and A. R. Williams in Journal ofthe Institute of the Rubber Industry. Vol. 8, No. 3, June 1974.Measurements of the wet grip were made for locked wheel slidingfriction.

Rolling resistance was measured on the rotary power loss machine.

The results obtained are shown in Table D below.

                  TABLE D                                                         ______________________________________                                                                Rolling resistance                                    Experiment  Wet grip -  (Calculated according                                 No.         (sliding friction)                                                                        to Equation 3.1)                                      ______________________________________                                        Comparison                                                                    S-1502      100         100                                                   Invention                                                                     11          118         99                                                    12          118         99                                                    13          112         100                                                   14          --          --                                                    14A         115         102                                                   15          101         98                                                    16          100         99                                                    16B         108         101                                                   16C         110         103                                                   17          116         99                                                    18          106         97                                                    19          101         98                                                    20           94         97                                                    21          119         98                                                    22          119         100                                                   23          117         99                                                    24          113         98                                                    26          113         95                                                    27          111         93                                                    28          110         96                                                    ______________________________________                                    

It will be seen from Table D that the tires tested have a goodcombination of wet grip and rolling resistance. It is believed that thebegin or tail end portion or portions of the polymer molecule having thestyrene-rich component makes a major contribution to the good wet gripand that the remainder of the molecule having a substantial vinylcontent and free from a styrene-rich component makes a majorcontribution to the good rolling resistance.

The invention is further illustrated by the following Experiments 29 to34, in which, unless stated otherwise, the proportion of ingredients inthe compositions is given in parts by weight.

The compositions of Experiments 29 to 31 include as their polymeringredient solution styrene-butadiene copolymers referred to below ascopolymers J and K, respectively. The bound styrene content, vinylcontent (expressed as a percentage by weight of the butadiene content ofthe copolymer) and molecular weight are shown in the following table,and both copolymers are polymers which have a linear structure.

    ______________________________________                                                  Styrene     Vinyl    Molecular                                                Content     Content  Weight ×                                 Copolymer (% IR)      (% IR)   10.sup.3                                       ______________________________________                                        J         23          33       450                                            K         24.8        30       429                                            ______________________________________                                    

That portion of the copolymers in which the butadiene has reacted by"head-to-tail" 1,4 polymerisation has been found largely to have a transconfiguration. The molecular weights referred to in the table are peakmolecular weights obtained by gel permeation chromatography usingpolystyrene as a comparative standard.

Elastomer compositions of the invention, suitable for use as tiretreads, have been obtained by blending together the followingingredients and vulcanising for 40 minutes at 140° C. in a steamautoclave.

    ______________________________________                                                    Experiment No.                                                    Ingredients   29          30      31                                          ______________________________________                                        Copolymer J   100.00                                                          Copolymer K               100.00  72.50                                       Dutrex 729 Oil                    27.50                                       Sulphur       1.75        1.75    1.75                                        Stearic acid  1.00        1.00    1.00                                        Zinc Oxide    3.00        3.00    3.00                                        75% BLE       2.00        2.00    2.00                                        C.B.S. accelerator                                                                          1.00        1.00    --                                          M.B.S. accelerator                                                                          --          --      1.00                                        N 375 carbon black                                                                          50.00       50.00   50.00                                       ______________________________________                                    

The compositions of Experiments 32 to 34 each contains as its polymeringredient a solution styrene-butadiene copolymer referred to below ascopolymer A, B and C respectively. The bound styrene content, vinylcontent (expressed as a percentage by weight of the butadiene content ofthe copolymer) and molecular weight are shown in the following Table,and they are all polymers having a linear structure.

    ______________________________________                                                  Styrene     Vinyl    Molecular                                                Content     Content  Weight ×                                 Copolymer (% IR)      (IR)     10.sup.3                                       ______________________________________                                        A         22          67       480                                            B         25          52       480                                            C         24.7        47       427                                            ______________________________________                                    

That portion of the copolymers in which the butadiene has reacted by"head-to-tail" 1,4 polymerisation has been found largely to have a transconfiguration. The molecular weights referred to in the table are peakmolecular weights obtained by gel permeation chromatography usingpolystyrene as a comparative standard.

Three elastomer compositions of the invention, suitable for use as tiretreads, have been obtained by blending together the followingingredients and vulcanising for 40minutes at 140° C., in a steamautoclave.

    ______________________________________                                                    Experiment No.                                                    Ingredients   32         33      34                                           ______________________________________                                        Copolymer A   100.00     --      --                                           Copolymer B   --         100.00  --                                           Copolymer C   --         --      100.00                                       Sulphur       1.75       1.75    1.75                                         Stearic acid  1.00       1.00    1.00                                         Zinc oxide    3.00       3.00    3.00                                         75% BLE       2.00       2.00    2.00                                         C.B.S. accelerator                                                                          1.00       1.00    --                                           M.B.S. accelerator                                                                          --         --      1.00                                         N 375 carbon black                                                                          50.00      50.00   50.00                                        ______________________________________                                    

The compositions of Experiments 29 to 34 have been found to have adesirable combination of wet grip and rolling resistance properties asshown in Table E.

                  TABLE E                                                         ______________________________________                                                     Wet grip -                                                                              Rolling resistance                                     Experiment   sliding   (calculated according                                  No.          friction  to Equation 3.1)                                       ______________________________________                                        S-1502       100       100                                                    (comparison)                                                                  29           110       93                                                     30           111       96                                                     31           123       110                                                    32           110       100                                                    33           118       99                                                     34           116       99                                                     ______________________________________                                    

EXPERIMENTS 35, 36 AND 37

Several other copolymers have been produced according to the conditionsreferred to in Table VIA, and their properties are given in Table VlB.In Experiments 35 and 36 isoprene is used in a way such that itpolymerises substantially entirely in the "tail" portion of thecopolymer--an example of the use with butadiene of a second conjugateddiene hydrocarbon.

The copolymers of Experiments 35 and 36 are begin tail polymers and thenature of the tail is determined largely by the batchwise charge. InExperiment 35 the diene component in the batchwise charge is halfbutadiene and half isoprene leading to a tail having a substantialisoprene content; in Experiment 36 the batchwise diene component iswholly isoprene leading to a tail substantially without butadiene.

In Experiment 37, as in Experiments 35 and 36,addition of monomers afterthe start of the reaction is carried out continuously.

                  TABLE VIA                                                       ______________________________________                                         Solution SBR - Polymerisation procedure                                      ______________________________________                                        Polymer batch size: 40.0 kg                                                                      Polymerisation temp:                                                          55° C.-60° C.                                Solvent: cyclohexane                                                                             Coupling agent: dimethyl                                                      adipate                                                    Final solids content: 20.0% w                                                                    Coupling time: 15 min.                                     Ortho dimethoxybenzene: 450 ppm                                                                  Total polymerisation time:                                                    120 min.                                                   s-Buli: 0.267 mol (kinetic MW,                                                                   Reactor - R 260/3501                                       before coupling: 150.000)                                                                        nominal capacity                                                         Sample number                                                                   10802      10803   10804                                      Conditions      35         36      37                                         ______________________________________                                        Batchwise charged                                                             Butadiene, kg   0.077      --      0.45                                       Isoprene, kg    0.077      0.154   --                                         Styrene, kg     0.92       0.92    2.76                                       1st Continuous addition                                                       Butadiene, kg   27.57              27.27                                      Isoprene, kg    --         6.89    --                                         Styrene, kg     8.28       2.07    6.44                                       time of addition, min.                                                                        80         20      80                                         2nd Continuous addition                                                       Butadiene, kg   3.08       20.68   3.08                                       Isoprene, kg    --         --      --                                         Styrene, kg     --         6.21    --                                         time of addition, min.                                                                        20         60      20                                         3rd Continuous addition                                                       Butadiene, kg, in 20 min.                                                                     --         3.08    --                                         ______________________________________                                    

                                      TABLE VIB                                   __________________________________________________________________________                                       Infrared Analysis                                                                  Butadiene                                    GPC - analysis                   portion                                      app.                                1.4                                                                              1.4                             Exp.                                                                             Sample                                                                            peak                                                                             MW × 10.sup.-3                                                                 CE,               Styrene,                                                                           1.2                                                                              trans                                                                            cis                             No.                                                                              No. BC AC     %  --M.sub.w × 10.sup.-3                                                         --M.sub.n × 10.sup.-3                                                         Q  %    %  %  %  Mooney                       __________________________________________________________________________    35 10802                                                                             220                                                                              585    57 402   215   1.88                                                                             23.9 47.7                                                                             29.7                                                                             22.6                                                                             44                           36 10803                                                                             370                                                                              1100   51 506   260   1.96                                                                             24.2 40.5                                                                             26.4                                                                             33.2                                                                             73                           37 10804                                                                             235                                                                              600    56 420   250   1.68                                                                             25.4 48.3                                                                             28.8                                                                             23.0                                                                             60                           __________________________________________________________________________

The copolymers of Experiments 35-37 have been used, as the tread portionof full size tires (155 SR13 SP4), and their wet grip and rollingresistance has been determined as follows:

Wet grip: using the internal drum testing facility at University ofKarlsruhe

Rolling resistance: using the rotary power loss machine referred toabove.

The results obtained were as shown in Table F below.

                  TABLE F                                                         ______________________________________                                                                        Rolling                                                                       Resistance (calculated                        Exp. Compound   Wet Grip Rating according to                                  No.  polymer   Peak   Cornering                                                                             Mean  Equation 3.1)                             ______________________________________                                        Comparisons:                                                                  --   S-1502    100    100     100   100                                       --   S-1712    108    104     106   103                                       --   OEP/LTP   107    100     103   115                                       35   10802     113    107     110   101                                       36   10803     120    116     118   99                                        37   10804     117    120     118   98                                        ______________________________________                                    

Having now described our invention what we claim is:
 1. A copolymerobtained by coupling together two or more component copolymers, each ofsaid component copolymers, which can be the same or different, being anelastomeric copolymer of an aromatic vinyl compound and a conjugateddiene, suitable for use in the tread portion of a pneumatic tire, saidaromatic vinyl compound being a compound selected from the groupconsisting of styrene, 1-vinyl naphthalene, 3,5-diethylstyrene,4-n-propylstyrene, 2,4,6-trimethyl styrene, 4-phenyl styrene,4-p-tolystyrene 3,5-diphenylstyrene, 3-ethyl-1-vinylnaphthalene, and8-phenyl-1-vinyl naphthalene, said conjugated diene being a compoundselected from the group consisting of 1,3 butadiene, 2-methyl-1,3-butadiene (isoprene), 1,3-pentadienes (piperylenes), 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-pentadiene, 2,3-dimethyl-1, 3-pentadiene, and2-phenyl butadiene, said elastomeric copolymer having a vinyl content ofat least 30% by weight and having a differential content of the aromaticvinyl compound such that in at least one of its end portions saiddifferential content changes in a portion of not more than 5% of thecopolymer chain (as determined by monomer conversion) from a first valueto a second value, said second value being not more than 98% and beingat least 25 percentage points greater than said first value; and saidportion lies within a terminal 10% portion of the copolymer chain (asdetermined by monomer conversion).
 2. A copolymer obtained by couplingtogether two or more component copolymers, each of said componentcopolymers, which can be the same or different, being an elastomericcopolymer of an aromatic vinyl compound and a conjugated diene, suitablefor use in the tread portion of a pneumatic tire, said aromatic vinylcompound being a compound selected from the group consisting of styrene,1-vinyl naphthalene, 3,5-diethylstyrene, 4-n-propylstyrene,2,4,6-trimethyl styrene, 4-phenyl styrene, 4-p-tolystyrene3,5-diphenylstyrene, 3-ethyl-1-vinylnaphthalene, and 8-phenyl-1-vinylnaphthalene, said conjugated diene being a compound selected from thegroup consisting of 1,3 butadiene, 2-methyl-1,3 butadiene (isoprene),1,3-pentadienes (piperylenes), 2,3-dimethyl-1,3-butadiene,2-methyl-1,3-pentadiene, 2,3-dimethyl-1,3-pentadiene, and 2-phenylbutadiene, said elastomeric copolymer having a vinyl content of at least30% by weight and having a differential content of the aromatic vinylcompound such that in at least one of its end portions said differentialcontent changes in a zone of not more than 21/2% of the copolymer chain(as determined by monomer conversion) from a first value to a secondvalue, said second value being at least 14 percentage points greaterthan said first value; and said zone lies within a terminal 10% portionof the copolymer chain (as determined by monomer conversion).
 3. Acopolymer according to either claim 1 or 2, in which the coupled two ormore copolymers are vulcanized.
 4. A copolymer according to claim 1 or2, in which said coupling is effected using a tetra-functional couplingagent.
 5. A coupled copolymer obtained by coupling together two or morecopolymers, each of said copolymers, which can be the same or different,being an elastomeric copolymer of an aromatic vinyl compound and aconjugated diene, suitable for use in the tread portion of a pneumatictire, said aromatic vinyl compound being a compound selected from thegroup consisting of styrene, 1-vinyl naphthalene, 3,5-diethylstyrene,4-n-propylstyrene, 2,4,6-trimethyl styrene, 4-phenyl styrene,4-p-tolystyrene 3,5-diphenylstyrene, 3-ethyl-1-vinylnaphthalene, and8-phenyl-1-vinyl naphthalene, said conjugated diene being a compoundselected from the group consisting of 1,3 butadiene, 2-methyl-1,3-butadiene (isoprene),1,3-pentadienes, (piperylenes),2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-pentadiene, 2,3-dimethyl-1, 3-pentadiene, and2-phenyl butadiene, said elastomeric copolymer having a vinyl content ofat least 30% by weight and having a differential content of the aromaticvinyl compound such that in at least one of its end portions saiddifferential content shows a sharp and substantial increase in thedirection of the outer extremity of said end portion, the content ofsaid aromatic vinyl compound in at least one of the end portions of saidelastomeric copolymer changing from a first value to a second value, thesecond value being less than 100% in which said coupling has beeneffected at a portion of each of the elastomeric copolymers of aromaticvinyl compound and conjugated diene other than at said at least one endportion.
 6. A copolymer according to claim 5, in which said aromaticvinyl compound is styrene.
 7. A copolymer according to claim 5 or 6 inwhich said diene is butadiene.
 8. A copolymer according to claim 5 or 6,in which said diene is isoprene.
 9. A copolymer according to claim 5, inwhich the copolymer is a styrene-butadiene copolymer.
 10. A coupledcopolymer according to claim 5, in which said elastomeric copolymer is astyrene-butadiene copolymer, whose percentage differential styrenecontent in said at least one end portion changes in a portion of notmore than 5% of the copolymer chain (as determined by monomerconversion) from a first value to a second value, said second valuebeing at least 25 percentage points greater than said first value; andsaid portion lies within a terminal 10% portion of the copolymer chain(as determined by monomer conversion).
 11. A copolymer according toclaim 10, in which said 5% portion is a terminal portion of thecopolymer chain.
 12. A copolymer according to claim 10 or 11, in whichthe percentage differential styrene content changes in a zone of notmore than 21/2% of the copolymer chain (as determined by monomerconversion) from a first value to a second value, said second valuebeing at least 14 percentage points greater than said first value; andsaid zone lies within a terminal 10% portion of the copolymer chain (asdetermined by monomer conversion).
 13. A copolymer according to claim10, in which said second value is at 0% monomer conversion.
 14. Acopolymer according to claim 5, which consists substantially wholly ofstyrene and butadiene.
 15. A copolymer according to claim 5, in whichthe bound styrene content of the copolymer is at least 10% by weight.16. A copolymer according to claim 5, in which said vinyl content is inthe range from 30% to 60% by weight.
 17. A copolymer according to claim5, which is linear.